At Rockefeller Center on Sunday, milling outside Christie’s sales rooms — where private clients sipped mimosas as they took in one of Monet’s grainstacks — people in the art world sounded guardedly optimistic about how the auctions will perform this week, after a period of uncertainty exacerbated by the contentious American presidential election, Britain’s “Brexit” vote in June and China’s slowing economy.

“There has been a lot of insecurity and it’s hard to say exactly what will happen,” said Jay Gorney, a collector, curator and former dealer, predicting that “good things will do extremely well.”

The sales of Impressionist, Modern and contemporary art that start Monday offer the first test of how the art market will react to a Trump presidency and whether it will continue a softening trend that, for the past year, has had potential sellers reluctant to consign their best works.

“If you’ve got something great, you don’t sell it because you’re uncertain what you’re going to get for it,” said J. Tomilson Hill, the vice chairman at the Blackstone Group and art collector, about the prevailing mood. “Sellers are largely sitting at their hands.”

The result is that each of the three major auction houses — Christie’s, Sotheby’s and Phillips — are entering this week’s sales with fewer trophies of more than $20 million than they have had in the recent past. Over all, the estimated sales in postwar and contemporary art are half what they were last year. The evening auctions of postwar and contemporary art at the three houses, for example, are expected to draw about $536 million, compared with $1.2 billion for the same auctions in November 2015.

“They gathered the best material they could with a lot of sellers cautious and not willing to commit,” said Neal Meltzer, an art adviser. “Supply is the issue more than demand.”

In the days before the sales, collectors, art advisers and auction specialists were pointing to encouraging signs, citing the postelection stock market highs, the post-“Brexit” London sales in June and Sotheby’s London auction of David Bowie’s art collection last week, which had a sell-through rate of 100 percent and set new top prices for 59 artists.

Moreover, many collectors are sanguine about the effects of Mr. Trump’s victory, both in the United States and around the world. “I feel great,” said the real estate developer Arnie Rosenshein. “I was for Donald Trump.”

Yet history suggests that single events rarely affect sales, art experts say. “The market has been pretty impervious to just about every event with the exception of the global meltdown of 2008,” said Robert Manley, who recently became Phillips’s new co-head of 20th century & contemporary art after 16 years at Christie’s.

Donald B. Marron, a financier and longtime collector, said he expected the auctions to be largely business as usual. “Clearly this is a surprise and in one sense makes everybody cautious until they see how everything works,” Mr. Marron said. “On the other hand, if you want to judge by the markets, less regulation is seen as positive. My guess is, it will be like most auction seasons: good pictures will do well.”

Despite the challenging timing of the sales, the auction houses have managed to squeak out a few prizes. Sotheby’s secured Edvard Munch’s 1902 painting, “The Girls on the Bridge,” one of only two in private hands with this subject matter that is expected to sell in Monday night’s Impressionism and Modern auction for more than $50 million. It is one of only three works in the sale with a guarantee — a pledged minimum price; the others are a Picasso and a van Gogh. (Sellers of guaranteed works typically receive all of the hammer price and part of the buyer’s fees, with the auction house sometimes making a minimal profit.)

The auction house also secured the collection of Steven and Ann Ames, New York arts patrons, for its Contemporary sale Thursday evening — albeit with the help of a $100 million guarantee. It includes two colorful paintings by Gerhard Richter (one from 1986 and one from 1988), each estimated at $20 million to $30 million.

Christie’s’ highlights include a 1977 Willem de Kooning work, “Untitled XXV,” one of the artist’s large “pastoral” paintings, which is expected to sell for around $40 million on Tuesday night, at its Postwar and Contemporary sale. On Wednesday night, Christie’s Impressionist and Modern star lots include the Monet grainstack from 1891, which is expected to sell for more than $45 million, and a 1961 example of Jean Dubuffet’s Paris Circus series, estimated at $15 million to $20 million.

Earlier the same night, Phillips has Mr. Richter’s “Düsenjager,” estimated at $25 million to $35 million, part of a small group of warplane pictures created between 1963 and 1964. (Paul Allen is the seller.)

After Sunday’s brunch preview at Christie’s, not everyone came away impressed. “The estimates are 40 percent lower than last year,” said Jacques Seguin, a Swiss collector. “The electricity is not there.”

As for the auction houses, they say they learned from last year, when they priced estimates too high, and they are hopeful that the certainty of the election’s outcome could make some buyers more confident about bidding.

“Some people are very happy with this result and will feel richer and more inclined to buy art, and some people will feel destabilized by the unknown and still more inclined to buy art,” said Simon Shaw, Sotheby’s worldwide co-head of Impressionist and Modern art. “There’s a lot of demand out there.”

So much demand, auction houses say, that in many cases sellers — including the undisclosed consignor of the de Kooning at Christie’s, for example — have declined guarantees.

“They’re not in need of selling,” said Brett Gorvy, Christie’s worldwide chairman for postwar and contemporary art. “They’d rather have the painting back” if it doesn’t sell.

Given a shortage of masterpieces, auction houses have been forced to get creative.

Phillips is selling Roy Lichtenstein’s “Nudes in Mirror” (1994), which was slashed by a deranged woman in 2005 while on exhibition in Vienna. Typically, an auction house would play down that kind of damage to a painting. Instead, Phillips has produced a separate section of the catalog detailing its history.

“It may not add to the value but it adds to the story,” Mr. Manley, of Phillips, said. “We’ll find out on Nov. 16 what the market thinks.”

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